ORACLE Data Guard : Query

select NAME, DB_UNIQUE_NAME, OPEN_MODE, PROTECTION_MODE, DATABASE_ROLE from v$database;

select THREAD#, max(sequence#) from v$log_history group by  THREAD# order by THREAD# ;
SELECT PROCESS, STATUS FROM V$MANAGED_STANDBY; (if no RFS process then add tnsnames.ora)

Set linesize 1000
col DESTINATION for a40 
col ERROR for a80
select dest_id, archived_seq#, status,destination,error from v$archive_dest_status;

Following query will identify all archivelogs applied to the standby:
select a.thread#, a.sequence#, a.applied from v$archived_log a, v$database d where a.activation# = d.activation#  and a.applied='YES';
select a.thread#, a.sequence#, a.applied from v$archived_log a, v$database d where a.activation# = d.activation#  and a.applied='NO';

SELECT * FROM V$ARCHIVE_GAP;
SELECT NAME FROM V$ARCHIVED_LOG WHERE THREAD#=1 AND DEST_ID=1 AND  SEQUENCE# BETWEEN 443 AND 446;

SELECT SWITCHOVER_STATUS FROM V$DATABASE;